Re: #u3s Relocating R5 #u3s

J68HZ
 

Well let’s not kid ourselves here.  The IRF parts and the RD parts are very (very very) different animals.  The IRF510’s dies are hard-on switches, meaning they saturate/ switch on hard and abruptly…they are designed that way because of their actual intended service as motor switches and power supply parts…. and that, in an RF world can cause harmonics, ringing, and trash.  It means you need to work extra hard to filter that stuff out of your output signal when you push them… and it’s frequency dependent.  The RD devices are made to be RF switches with a smoother approach on the curve at full on.  Much less trash generated.  You already know about the capacitance issues.

 

 

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From: QRPLabs@groups.io [mailto:QRPLabs@groups.io] On Behalf Of ajparent1/KB1GMX
Sent: Saturday, August 10, 2019 5:59 PM
To: QRPLabs@groups.io
Subject: Re: [QRPLabs] #u3s Relocating R5

 

for a 5W amplifier the RD15HHF or RD16VHF is a over sized device.
There are also RD06 parts that might be a better match.  Or better
yet MRF136.

As to bias, this is to be a class C or maybe E amplifier.  Two things
have to exist for that enough drive to saturate the device and also 
drive it to cutoff.  The yabut is that Most mosfets have a gain vs
bias curve that forces some current flow to get reasonable operation.
So the gate bias voltage (NOT CURRENT) needs to be sufficient for
a small amount of drain current (under 50ma!) or you moving from
class C to class B or even class A.   That move impacts gain,
power out and output and input circuits for stability and power.

So dumping the supposedly better RD part usually is more illusion
that actual performance.  Those that claim improvement often had
other issues in the works or due to the significantly different
characteristics they changed the amp in ways they do not realize.
At RF there are few (nearly none) drop in improvements.

Bottom line is to get 5W out you need enough power in and about
200-500mw is enough depending on circuit and frequency.

FYI I use IRF510s and have RD15 and RD16 parts and generally I get
more out of the IRF510s as I can run higher voltages (up to 28V) that
would destroy the RD part (designed for 13.8V nominal, 16V max use).
As to the claim the IRF is not a good HF part the QRPlabs amp runs
to 70mhz at acceptably reduced gain (could be optimized) and solid
to 30mhz.   I have two amps of my design that are for 6M use one is
20W class and the other using 8 of them does 220W at 30V.  I also
have refitted a 2006 6M SSB radio that was then designed for 1W
based on junkbox final and that is now a nice 2.5W using the
IRF510 and of course the right circuit to extract that power.
Could it do more yes, but the existing driver is the limiting factor
not the output and I didn't want to change that.

The only place the RD parts have been better for me than IRF510
is at 144mhz  and up but I can get more power per $ using MRF
series parts.  But then we are talking 100++W levels and 30-50$
transistors.

Allison


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